FIGS. 5 to 7 show an example of a conventional commonly-used end mill. FIG. 5 shows an external appearance of the end mill. FIG. 6 shows an aspect during cutting work by the conventional end mill. FIG. 7 shows a cross-sectional shape of a worked surface of a work material formed by cutting work of the conventional end mill.
The end mill 500 illustrated in FIGS. 5 to 7 has a shank 108 on the end side of a mill body 105 and six spiral peripheral cutting edges 103 formed on the tip side. A peripheral rake face 101 is formed on the front side of each peripheral cutting edge 103 in the rotating direction R and a peripheral flank 106A is formed on the rear side of each peripheral cutting edge 103 in the rotating direction R. The typical usage manner of this end mill 500 is to allow the end mill 500 to remove part of a work material P to obtain a target shape by traveling along a side surface of the work material P while rotating (direction of arrow R) and giving a predetermined cutting to the work material P. At the time of such cutting work, since an undulation of the worked surface is caused by an elastic deformation of the end mill 500, such cutting work results in an accuracy defect for cutting work requiring a high-precision flatness and straightness in the axial direction.
Hereinafter an elastic deformation of the end mill and an undulation of the worked surface will be described. A commonly-used end mill 500 shown in FIG. 5 has spiral peripheral cutting edges 103 on the outer circumferential surface of a cylindrical mill body 105 and radial cutting edges on the tip of the mill body 105. This end mill 500 has a function of removal processing on the side surface and the bottom surface of the work material simultaneously by coming in contact with the work material while rotating. Since the end mill 500 overhangs in a cantilever state due to the machining process of the end mill and thus receives a load (hereinafter, cutting resistance) during the removal processing to the work material P in the lateral direction, the end mill 500 causes its deflection. The plurality of peripheral cutting edges 103 of the end mill 500 are separated from each other in the axial direction. Hence, the number and location of the cutting points varies in accordance with the rotation phase, and thus the deflection of the end mill also changes.
Hereinafter, description will be made with reference to FIG. 6(b) in which six peripheral cutting edges 103 of the end mill 500 are developed on a plane. When the rotation phase of the end mill 500 is at the position A in FIG. 6(b), the end mill 500 and the work material are in contact with each other at three points (circle marked positions in FIG. 6(b)) and the contact positions are located on the tip side of the end mill 500. Next, when the end mill rotates to the position of rotation phase B, though the end mill 500 and the work material have three contact points, the contact positions are closer to the base (closer to the shank 108) of the end mill 500 and the deflection of the tool reduces. At the position of rotation phase C, the end mill 500 and the work material have two contact points, and since the contact positions become further closer to the base, the deflection further reduces. At the position of rotation phase D, the contact state becomes similar to the rotation phase A and the deflection increases. In reality, since the rotation speed of the end mill 500 is extremely high compared to its traveling speed in the horizontal direction, the phenomena of the above rotation phases A to D continuously appear on the cross section of the work material P in the axial direction V, resulting in an undulation on the worked surface M of the work material P as shown in FIG. 7 and an undulation height HA that is a difference between the highest position and the lowest position based on the axis V increases.
For cutting work requiring a high-precision flatness and straightness in the axial direction, the above-described undulation becomes a problem. Hence, to improve the straightness of the worked surface of a work material, proposed is a method in which the diameter of the tool is continuously varied in the axial direction to offset a predicted undulation based on previous prediction of the position and height of the undulation on the worked surface corresponding to the rotation phase of the end mill (for example, refer to Patent Literature 1).